When performing machine tool setup and maintenance operations, it is frequently necessary to use a position transducer or the like to “sweep” a surface and establish its relationship to a machine datum. A machine operator or maintenance technician typically attaches a position indicator to one portion of a machine, and then “sweeps” the position transducer over the object of interest, while observing the “indicator run-out.” If necessary, multiple adjustments and rechecks are made to the position of the object of interest to obtain the desired alignment condition.
Similarly, when machining a workpiece, it is frequently necessary to place the part in a particular position in relation to a fixture or a reference location, such as the center or plane of rotation of a spindle. A conventional approach for accomplishing this task is for a machine operator to manually measure the misalignment of the part and then move it into the correct position using a series of incremental steps. When a workpiece is being aligned on a spindle, the machine operator rotates the spindle while measuring the part “run-out” with a position transducer and “taps” the part with a mallet or the like to correct any misalignment. If non-figure-of-revolution surfaces are being aligned, then the machine's linear axes are used to “sweep” the surface into alignment.
While the above operations may usually be done accurately in an open machining environment, they may be very difficult and time consuming when access to the workpiece is constrained by the machine size or when the machine is enclosed in a glovebox or the like. In such a situation, the machine operator has limited visibility and manual alignment operations are difficult to perform, especially when done at arm's length, while wearing glovebox gloves. This is problematic.
Thus, centering a part on a spindle for precision machining, for example, is a tedious, time-consuming task under even the most ideal circumstances. Currently, a skilled operator must measure the “run-out” of a part using a displacement gauge or the like, then tap the part into place using a plastic or rubber hammer or the like. What are still needed in the art are systems and methods for automatically centering a part on a vacuum chuck, for example, with initial “run-out” that exceeds acceptable limits. In order to provide such systems and methods, one must measure the magnitude and direction of the radial “run-out” and then actuate the part until the part and spindle centerlines are within acceptable tolerances of each other. The “run-out” may be measured with either a position transducer mounted to a machine axis or an electronic gauge for example. The part is tapped into place with a linear actuator driven by a voice coil motor, for example. As a result, a part may be automatically corrected without significant human intervention.
As alluded to above, one of the most prevalent challenges in manufacturing precision parts is the alignment of a part on a holding fixture, such as a vacuum chuck. In a typical diamond turning operation, for example, an operator is required to manually place the part on the vacuum chuck then center the part. Also, during the fabrication of a two-sided part, for example, precision alignment is needed to align features on one side with features on the other side. For example, a hemi-shell must be machined such that the inner contour (IC) and outer contour (OC) are concentric. This requires that the part be transferred from one chuck designed for IC machining to another chuck designed for OC machining. This transfer process is usually done by a skilled operator who is responsible for removing the part from one chuck, then replacing and centering the part on the other chuck. In an ideal situation, an operator can center a part to within about 5 μm of the spindle centerline in about 15 minutes; however, in a limited access, limited visibility situation, the alignment operation may take much longer. The centering process involves measuring part “run-out” with a displacement gauge or the like, such as a Federal gauge or linear variable differential transformer (LVDT), then tapping the part with a plastic or rubber hammer or the like and repeating until the “run-out” is acceptably small. Automating the centering operation would advantageously reduce the overall human effort required and produce more accurate and repeatable positioning. The result would be a significant cost savings and improved part accuracy.
The behavior of the friction force, which holds a part on a vacuum chuck, is pivotal to the development of an improved realignment technique. Countless researchers have characterized the behavior of the friction force for a variety of applications—modeling it, studying the interplay of static and viscosity, analyzing its non-linearity as a function of velocity, explaining the behavior of the friction interface prior to slipping, etc. It has been suggested that the friction interface behaves as if the two relevant surfaces are coated with asperities that act like springs, allowing the two relevant surfaces to displace slightly under a force that is lower than static friction. When observed, this behavior is especially important in describing friction on the micrometer scale.
Work that applies friction models specifically to actuation against friction force and automated part alignment has also been attempted previously, including involving a tapping method of actuation. Some of this work has been based on a vertical actuator used to adjust the position of a workpiece on a horizontal rotary stage, where the normal force is generated entirely by the weight of the part—with and without the use of an impulse generated by an electromagnet, for example, which is incapable of producing a force output that is constant with stroke. The result has been poor control of the applied impulse, and an unnecessarily complicated mounting system when the technique is used with different sized parts. The work of the present disclosure is based on a horizontal actuator that may be mounted in a machine's tool holder.
Other work has attempted controlled impulse manipulation by tapping a part and using position feedback to ensure that the part approaches a commanded position. Additionally, the controllability of a micropositioning system has been explored using three separate tapping actuators on the same part. Further, tapping controlled by fuzzy reasoning and feedback from an eddy current probe has been employed to position a part. Others have attempted to align parts using high frequency, low amplitude vibrations generated by piezoelectric actuators or the like.
While such research has endeavored to position a part held in place by friction, many of the methods utilized require precise positioning of the realignment actuator, which operates over a short range. The long range actuators utilized by others do not provide a repeatable impulse through the required actuator stroke. This shortcoming makes it necessary to control the position of the actuator with respect to the part, or to use real-time feedback to determine the needed actuation power. Such needed control adds complexity, especially if the part moves substantially during a centering process, for example.
Thus, what is also still needed in the art is a long range actuator that may be used for the precision alignment required for the production of any part. The design of the system must be motivated by experimental studies of the friction interface and elastic collision between the actuator head and part. Analysis also must be given of several different methods for automating “run-out” measurements. Depending on the positioning accuracy required and available sensors, one method may be preferable to another.